It has been common practice for some time to utilize electrical detection equipment to determine the exact location and depth of buried metallic lines such as underground pipes or cables, particularly when construction work is in progress and there is a known risk of damaging or destroying those critical assets. According to the conventional practice, an alternating current is induced in the line at a first point along its length, and at a second point along its length a measurement above ground is made of the electromagnetic field which then appears as a result.
As is well known, at the first work station the alternating current is induced in the line at a frequency which may typically be 100 Kilohertz, and at the second work station a search coil associated with a receiver tuned to the same frequency is moved about in order to accomplish the measurements. Depending upon the orientation of the search coil, its lateral movement will produce either a maximum or a minimum reading indicative of the lateral location of the line. Other measurement techniques are then used to determine the exact depth of the buried line at the second location.
Any electromagnetic field energy which is transmitted above ground and picked up by the receiver tends to interfere with the accuracy of the desired measurements. However, the underground transmission of energy is significantly more efficient than the transmission above ground, since it is based upon electrical current flowing in a continuous, electrically conductive medium.
For effective field work it is common practice to utilize a portable, battery-powered transmitter at the first work station. The maximum distance from the transmitter that the receiver can provide reliable measurements is, in general, proportional to the output energy from the transmitter that is successfully induced into the underground line. Thus, the efficiency with which the transmitter utilizes the battery power is very important.
Sometimes the metallic line is buried quite deeply, and since the transmitter is positioned only on the surface of the ground, a strong signal is required in order to penetrate the full depth of earth. Not only is it essential to generate a strong transmitter signal, but the magnetic flux pattern should be widely dispersed so as to reach a deeply buried line.
Thus, it has become important in this technology to achieve as strong and effective an induced signal as possible, consistent with transmitter equipment that is easily portable and is powered by batteries which are part of the transmitter equipment.